With respect to the economic feasibility of commercial aircraft, it is important to design as much of the available fuselage volume as possible for accommodating payloads or, in other words, to prevent unnecessary voids in the fuselage. One particular problem arises when—for example, in modern wide-body aircraft—the cargo space underneath the main deck is larger than necessary for transporting the luggage of the passengers. Although a sufficient volume for additional freight containers is available in this case, the revenues are reduced in comparison with the more economical transport of passengers.
In wide-body aircraft, in particular, with an average fuselage diameter in excess of 5 m, a lower deck of corresponding size is provided underneath the very wide main deck that approximately lies in the center of the fuselage cross section. This geometry also ensures the accessibility of the lower deck, for example, if part of the cargo space floor is lowered and forms a center aisle. A lower deck that can be utilized by persons is known and described, for example, in DE 44 16 506 C2.
According to DE 43 13 592 C2, passengers can be accommodated in the lower deck due to the fact that energy-absorbing structural elements are arranged underneath the lower fuselage shell in order to protect the passengers by reducing the impact forces in case of a crash. This publication discusses a conventional low-wing commercial aircraft, in which the utilization of the lower deck is significantly restricted due to the space requirement of wing torsion boxes and landing gears. In such instances, the lower deck is not realized continuously, i.e., passenger areas situated forward and rearward of the wing and the landing gears are not interconnected.